electronic troubleshooting

Electronic Troubleshooting

Chapter 7Transformer-Coupled Circuits

Transformer-Coupled Circuits• Characteristics

• Some times used to couple stages of a circuit• Offers some advantages

• When it is necessary to make either low or high impedances appear as the opposite

• When it is desirable to only amplify a narrow band of frequencies

• Transformers transfer energy, thus: Pprimary = Psecondary • Ideal ones match - real ones have some losses

• Aspects covered• Untuned Interstage Transformer Coupling• Transformer Coupled Amplifier• Test & Troubleshoot Transformer Coupled Amplifiers• Tuned Transformers• Amplifiers Working into Parallel-Tuned Circuits

Untuned Interstage Transformer Coupling

• Characteristics• Transformers designed to operated over a wide range of

frequencies are called untuned• Example range of freqs – the audio spectrum 20 -20kHz• Usually have lower losses than power transformer

• Example: Untuned 4:1 step down transformer

Untuned Interstage Transformer Coupling• Characteristics

• Example: Untuned 4:1 step down transformer• n = Np/Ns = vp/vs

• n= turns ratio; Np= number of primary turns

• Ns= number of secondary turns; vp= primary voltage

• vs= secondary voltage

• If the secondary has less turns – Step-down transformer• If the primary has less turns – Step-up transformer

• Example problem• For the transformer on page 165

• Find the turns ratio and secondary voltage

450

200

s

p

N

Nn 3

124 s

ss

p vv

v

v

vn


• Characteristics• Secondary/Primary Current and Power

• is = vs /RL

• For the previous

example problem

• Pprimary = Psecondary


example problem

mAV

R

v

L

s 20015

3is

sspp vivi p

ssp v

vii

n

ii sp

mAmA

n

ii sp 50

4

200


• Characteristics• Reflected Impedance

•


example problem

• Sub for ip

24050

12

mA

V

i

vZ

p

pp

p

pp i

vZ

ni

v

i

vZ

s

p

p

pp

L

s

R

vsi

reflectedLLLs

p

s

p

p

pp rRnnRnnR

v

vn

i

v

i

vZ 2*

Transformer Coupled Amplifier• Key Aspects

• Amplifiers gain is dependent upon the load resistance seen on the output. Distortion also is dependent.

• AV= rL/re

• Too A small load resistance

causes distortion• Transformers can make a

small load appear to have

much higher resistance

Lreflected Rnr 2

Transformer Coupled Amplifier• Key Aspects

• Sample Circuit• Given: re = 12Ω, Vin = 5mV

• Find: n, rref , AV , vL 625

150

s

p

N

Nn

1800

5062

2

reflected

reflected

Lreflected

r

r

Rnr

15012

1800

e

LV r

rA

mVAvv ViC 750mVv

v

mV

v

vn s

ss

p 125750

6

Test & Troubleshoot Transformer Coupled Amplifiers

• Key Aspects• Typical collector Voltage

• Without signal• Almost equal to Vcc • Very low DC winding resistance• Thus very small voltage drop on

the winding • Goes higher than Vcc with input

• Inductive kickback » Changing primary current causes

generation of voltage that adds to

the source voltage» Can be very large with an open secondary

Test & Troubleshoot Transformer Coupled Amplifiers

• Key Aspects• Typical problems

• Open primary winding in the previous circuit• No output at Vc

• Open Secondary• 0V across the load• High voltages across the primary (at the start also kickback)

• Shorted Secondary or Primary • Reflected impedance near zero• Almost no output AC signal – Dramatic drop in Av

• Transformer Replacement• Use exact replacements if possible• Else match the turns ratio and physical size – same size usually relates

to freq response characteristics

Tuned Transformers• Key Aspects

• Covered Items• Parallel Resonance• Tuned transformers

• Parallel Resonance• Characteristics

• Parallel circuit with and inductor and a Cap» Often called a Tank Circuit

• At a Freq where XL = XC the circuit is at resonance

• Resistance or Impedance of the Tank Circuit => Zt = Q XL

» Where Q = XL / Rc , XL =2πfL

LCfr 2

1

faradsceCapaciC

henriesceInducCoilL

Freqsonantf

Where

r

tan

tan

Re

:Note the DC resistance of the Coil must be less than 1/10 of XL at resonance


• Parallel Resonance• Characteristics

• Resistance» Curve to the right

• Equivalent of the inductor coil» Has an ideal inductor in series with a resister that = the DC resistance of the inductor

• Example Problem• Given: Circuit on the previous slide, L=2mH, C=0.003µF, Rcoil = 20Ω

• Find: fr , Q, and Zt

KHzLC

fr 9.6410310228.6

1

2

193


• Parallel Resonance• Example Problem

• Given: Circuit on the previous slide, L=2mH, C=0.003µF, Rcoil = 20Ω

• Find: fr , Q, and Zt

81729.6428.62 mHKHzfLX L

8.4020

817

c

L

R

XQ

KXQZ Lt 4.338178.40


• Tuned transformers• Many coils can have their inductance adjusted in order to adjust the

resonant frequency• They utilize threaded cylinders made of iron (called slugs)

• The amount of cylinder in the coil can be adjusted • By changing the amount of Iron in the coil the inductance is

adjusted» Thus the resonant frequency

• Adjustment of coil slugs should be minimized• Usually can only be adjusted a

few times without damaging t

he coil

Tuned Transformers• Typical Circuits

• Characteristics• Like with untuned transformers a small load on the secondary is

reflected into the primary as a much higher impedance• Thus increasing the gain of the driver

stage• The gain curve with

respect to frequency looks

like the Impedance curve

on the previous slide• Only a selected

small range of freq

are amplified to a

significant level


• Characteristics• The gain curve with respect to frequency looks like the Impedance

curve on the previous slide• For example the two IF amplifiers shown in Figure 7-7 on page

173 typically only have a bandwidth of 10KHz around 455KHz• Bandwidth review

• Packaging – Varies. The ones used in the IF Amp on page 173• Have the transformer and tank Cap in a grounded tin can

» Adjusting slug is accessed through a hole on the top


• Tuning process• Varies per manufacturer• Also called Alignment• Some use O-Scopes others can use DC voltmeters on the AGC circuit

• Same end result – Tank resonant frequency is adjusted

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